As to a recording/reproducing device for recording/reproducing information to/from an optical recording medium such as an optical disc, there are two contradicting requests: a request for downsizing the recording/reproducing device and reducing the costs in fabricating the recording/reproducing device; and a request for increasing recording density of information.
Therefore, in order to downsize the recording/reproducing device, it has been considered to downsize an optical pickup device in the recording/reproducing device, and there have been proposed various methods for constituting an optical integrated module in which a plurality of elements in the optical pickup device are integrated.
An example of the optical integrated module is a hologram laser in which a semiconductor laser chip, a light-receiving element chip, and a hologram device are integrated in a single package. With reference to FIGS. 7(a) to 7(c), FIGS. 8(a), and 8(b), the following explains a hologram laser and an optical pickup device using the hologram laser. FIGS. 7(a) to 7(c) schematically illustrate the hologram laser. FIG. 7(a) illustrates the front of the hologram laser, FIG. 7(b) illustrates the left side of the hologram laser, and FIG. 7(c) illustrates the bottom of the hologram laser. Further, FIGS. 8(a) and 8(b) schematically illustrate an optical pickup device including the hologram laser in FIG. 7. FIG. 8(a) illustrates the front of the optical pickup device seen from an optical disc (not shown). FIG. 8(b) illustrates the side of the optical pickup device.
As illustrated in FIGS. 7(a) to 7(c), a hologram laser 7 includes: a semiconductor laser chip 71; a light-receiving element chip 72; a metal stem 73; leads 74; a cap 75; and a hologram glass 76.
The light-receiving element chip 72 in the hologram laser 7 includes: a light-receiving section including a plurality of photodiodes; an integrated circuit section for converting current outputs of the photodiodes into voltages, respectively, and for amplifying the current outputs; and a plurality of pads via which signal outputs of the integrated circuit section, a power source, and a ground terminal are connected with an outside. Each pad of the light-receiving element chip 72, and an anode and a cathode of the semiconductor laser chip 71 are connected with the leads 74 via metal wires in the cap 75.
As illustrated in FIGS. 8(a) and 8(b), the optical pickup device 8 includes components such as: the hologram laser 7; a collimating lens 81; a right-angle mirror 83; an objective lens 84; a flexible substrate (not shown) including a connection terminal section connecting to an external circuit. These components are provided in a housing 85 made of metal.
Laser light (emitted light) 77 emitted by the semiconductor laser chip 71 passes through the hologram glass 76, and then focuses on a recording surface of an optical disc 90 via the collimating lens 81, the right-angle mirror 83, and the objective lens 84 which are disposed outside the hologram laser 7. Reflected light 78 from the recording surface of the optical disc 90 tracks back and reaches the hologram glass 76. A hologram pattern is formed on an upper surface of the hologram glass 76, and diffracts the reflected light 78 from the optical disc 90 so that the reflected light 81 is converged onto the light-receiving element chip 72.
By using such hologram laser in which a semiconductor laser, a light-receiving element, and other optical components are integrated, it is possible to downsize the optical pickup device.
Further, as a method for integrating elements constituting an optical pickup device, other than the method in which the hologram laser is used, there is known a method for using an IC chip including a light source and a light-receiving element, namely, a laser coupler in which the light source and the light-receiving element are integrated (see Document 1: Japanese Unexamined Patent Publication No. 298172/2003 (Tokukai 2003-298172;published on Oct. 17, 2003) for example). Further, there is proposed a complex optical unit in which a light-emitting member (two-wavelength laser diode), a light-receiving member (semiconductor); and other components are integrated (see Document 2: Japanese Unexamined Patent Publication No. 339182/2001 (Tokukai 2001-339182; published on Dec. 7, 2001) for example).
On the other hand, in order to increase recording density of information in an optical recording medium, there is proposed multi-layering of a recording layer of the optical recording medium or increasing the number of apertures (NA) in an objective lens.
At that time, it is necessary to cancel stray light from other recording layer of the optical recording medium and/or to correct aberration of an optical system.
As a result, the amount of information signals is increased. A lot of terminals for signal outputs are necessary, accordingly. Generally, the number of terminals for signal outputs in a light-receiving element is 8 or so. In consideration of a power source, a ground terminal, a reference voltage input terminal, and a gain switching input terminal, the total number of terminals is 12 or so. In consideration of terminals for signal outputs used for cancellation of the stray light, correction of aberration of the optical system, and other additional functions of an optical pickup device, the total number of terminals in the light-receiving element is 20 or more. As the number of terminals is increased in this way, the number of wires connected with the terminals is also increased.
In view of the above, Document 1 proposes a method for arranging the laser coupler so that (i) input/output terminals are disposed on three sides of a bottom surface of the laser coupler and (ii) wires are drawn out of the fourth side of the bottom surface via a flexible substrate.
Document 2 proposes a method for providing external connecting terminals on both sides of a packaged light-receiving element, and for connecting the terminals with a flexible substrate.
However, in the method of Document 1, a semiconductor laser chip and a light-receiving element chip, each of which is in a bare chip state, are provided in the laser coupler so that the size of the laser coupler is downsized. As such, if a part of the chip has some defect, then the entire hologram laser or the entire laser coupler must be disposed of. This causes much loss of components.
To be specific, generally, it is difficult to inspect characteristics of a semiconductor laser chip or a light-receiving element chip each of which is in the bare chip state. Therefore the characteristics of the semiconductor laser chip and the characteristics of the light-receiving element chip must be inspected after the hologram laser or the laser coupler is made and the optical integrated module is fabricated. As a result, in the conventional method, if the characteristics of a semiconductor laser chip or the characteristics of a light-receiving element chip is inspected and judged to be defective, an entire optical integrated module must be disposed of. This causes mush loss of components.
Further, because it is difficult to adjust a mount position of the laser chip during fabricating the optical integrated module, it is necessary to design an optical system such as a hologram glass and a light-receiving element so that the hologram glass and the light-receiving element is most suitable for the size of the laser chip and radiation angle characteristics of the laser chip. In a case where a laser chip needs to be replaced with another laser chip with higher performance which is later developed and available, if there are differences in the size and/or characteristics of the laser chips, then optical design must be entirely reconsidered each time such a replacement is performed. This causes development time to be longer.
Further, in a case where a light source and a photodetector are integrated, it is difficult to carry out a positioning of the light source and the photodetector after the optical integrated module is fabricated.
Further, in the method of Document 2, although a packaged light-receiving element is used, the light-receiving element must be fixed to the main body of an optical integrated unit, after the light-receiving element is attached onto a reinforcing plate and is then combined with a flexible plate. As a result, fabrication is troublesome. Besides, the light-receiving element and terminals for the semiconductor laser do not exist on the same surface and therefore the flexible substrate must be bent in a complex manner, which makes working efficiency low.